1. Field of the Invention
This invention relates to a seismic data acquisition system and more particularly to a seismic data acquisition system wherein command, data and status signals are transmitted via optical fiber cables.
2. Description of the Prior Art
Seismic data acquisition systems use detector arrays to sense ground motion directly on land or in water. The generated signal is carried to an amplifier by a twisted pair of wires. Most acquisition systems have a large number of channels. Each channel receives a signal from a detector array by a twisted pair of wires. In a commonly used prior art system, the pair of wires are in a multi-conductor cable. The signals are in general very weak. They typically vary from a few millivolts to less than one microvolt. These multi-conductor cables have the following problems associated with them:
1. Signal deterioration due to cable length PA1 2. Laying out and picking up the cable is one of the larger labor costs in land data acquisition. PA1 3. Broken wires in the cable result in a large percentage of lost operating time. Finding the point in the cable where the wire is broken can be very time consuming. These problems are increased as more conductors are included in the cable and as the cable length is increased. Current trends in the industry indicate there will be longer cables and increasing numbers of channels. PA1 (a) supplying power for the transmitters PA1 (b) providing modulators with the dynamic range requirement for seismic data is nearly impossible PA1 (c) there is radio frequency interference from nearby transmitters and from static from thunderstorms PA1 (d) a separate carrier frequency is required for the transmitter of each DAU. PA1 (a) the DAU includes complex equipment and requires excessive power PA1 (b) the magnetic record from each magnetic recorder has to be collected and transcribed to a multichannel format PA1 (c) there is no monitor of system operation. A failure at a detector array will not be detected until the magnetic media is reproduced. PA1 (a) the radio frequency spectrum is crowded with users PA1 (b) the required high data rate forces use of high frequency which involves line of sight transmission problems PA1 (c) the available bandwidth is not adequate for systems of several hundred or more channels PA1 (d) any form of electromagnetic radiation can interfere PA1 (e) the power requirements are excessive PA1 (a) the tape cassette cannot record enough data for a typical operation PA1 (b) there is no monitor of system operation PA1 (c) the tape is exposed to a wide range of deleterious temperature variations PA1 (d) gathering up the tape cassettes and transcribing them PA1 (a) line attenuation increase drastically with higher frequencies PA1 (b) high frequencies travel faster than low frequencies, creating dispersion.
(a) long cables cause a loss of high frequency response due to distributed capacitance. PA2 (b) cross-talk between channels in the cable exists PA2 (c) there is power line frequency pickup interference PA2 (d) there is interference from radio transmitters PA2 (e) there is static pickup from thunderstorms
To overcome some of these problems, radio telemetry in analog systems is used. A digital acquisition unit (DAU) is used at each of a plurality of locations where one or more motion sensors are connected to the DAU. The DAU amplifies signals received from the sensors, modulates the signals and sends them via its own transmitter, back to a master station. The following problems existed with this system:
Another prior art system is an analog system which uses a magnetic recorder at each DAU. A radio receiver receives control commands from the master station. This system greatly reduces the required radio frequency bandwidth, but has the following problems:
In recent times, virtually all seismic operations use digital systems. The analog signal, after amplification and filtering, is converted to a digital format. Digital data is then recorded on magnetic tape.
One such prior art system is a radio frequency telemetry system which involves a radio receiver that receives commands from a radio transmitter. The transmitter transmits data to a master station. Time division multiplexing permits sharing one carrier frequency with a number of data channels. This prior art system has the following problems:
Another system that has been configured is one that involves a number of DAU's which have radio receivers to receive commands from a master station, an analog to digital converter and a tape cassette for recording the digitized data. This system has the following problems:
Also considered has been a digital system which involves wire line telemetry. In such a system, the wire line is intended to be less expensive and to involve less labor to deploy than the above described multi-conductor cable system. Some of these telemetry systems use coaxial cable, some use twin lead and others use a twisted pair. A wire line has a constant db loss per unit length. A wire line can only compete with a radio for short distances. For longer distances, repeaters must be used. These systems involve the following problems:
The above problems limit the length of line that can be used and force a fairly low data rate. The low data rate does not permit large numbers of channels unless multiple lines are used which introduces the problems of the multi-conductor cable. The use of a repeater in each DAU greatly reduces the above problems. The cable length is measured from one DAU to another. The data rate can be increased, allowing more channels. Also, a simple twisted pair can be used.
In the system of this invention, the fiber optics cable overcomes all of the problems identified above except that the attenuation is high. Quartz fibers now available have 20 db loss per kilometer. However, using each DAU as a repeater (as indicated above in the wire line telemetry system) solves the attenuation problem. The optical path passes extremely high bandwidths, permitting very high data rates and large numbers of channels. Using fiber optics also immunizes the system against electromagnetic radiation. There is no lightening hazard and the system is small and lightweight, reducing deployment costs.